What is 4D-CT?

4D-CT includes image sets in three planes (axial, coronal, sagittal) from the angle of mandible to the mediastinum (1). The “fourth” dimension of 4D-CT is the perfusion information derived from multiple contrast phases. It is most commonly performed with three phases: non-contrast, arterial, and delayed phase imaging (1, 2).

Findings of adenoma or hyperplasia

An oval shaped enhancing mass with low attenuation on non-contrast phase relative to thyroid, greatest attenuation in the arterial phase, and rapid washout of contrast in the delayed phase. There are variations in enhancement patterns and helpful morphological findings such as the polar vessel (1, 3, 4).

Do I really need all those imaging phases?

At least 3 phases are required because:

Only 19% of parathyroid lesions are higher attenuation than thyroid on arterial phase. Other lesions (such as the imaging example here) could be missed without delayed and non-contrast phases (5).

23% of adenomas have the similar enhancement to the thyroid on both arterial and delayed phase and could be missed without the non-contrast imaging (5).

How does 4D-CT compare to scintigraphy and ultrasound?

Several studies have found superior sensitivity of 4D-CT over scintigraphy in the workup of primary hyperparathyroidism.

Other advantages:

Difficult subgroups - 4D-CT is superior for subgroups of patients with small lesions, lesions in ectopic sites and multigland disease (2, 7, 8).

Surgical planning - 4D-CT assists with surgical planning by providing high-resolution images in multiple planes. The size of the adenoma, polar vessel to the adenoma and surrounding structures are better seen on CT than other modalities. These are all important information for planning minimally invasive surgery.

Grading diagnostic confidence – Rather than just stating a study is positive or negative, a combination of imaging findings allows radiologists to provide an estimation of degree of diagnostic confidence (1, 3-5). Diagnostic confidence has become increasingly important in pre-operative planning for minimally invasive parathyroidectomy, in which all four parathyroid glands are not visualized intra-operatively. High diagnostic confidence on 4D CT may also obviate the need for additional imaging, thereby reducing the cost of diagnostic evaluation.

Cost-effective – Two studies using decision models have shown that the use of 4D-CT as a second line investigation after traditional modalities fail to localize the lesion is more cost-effective than other imaging protocols (single modality or combination of ultrasound and scintigraphy) (9, 10). This finding is due to the fact that improved localization allows minimally invasive surgery to be performed which is a less costly treatment than bilateral neck exploration and failed surgery.

What about the risk from radiation exposure?

4D-CT has higher radiation dose than scintigraphy, but patients who develop primary hyperparathyroidism have a mean age in the 5th and 6th decades of life and have a lower risk for stochastic effects from radiation exposure (11, 12). A study comparing radiation dose from 4D-CT to scintigraphy found the effective dose was 28 mSv for 4DCT compared to 12 mSv for scintigraphy (13). However, in the exposed standard patient (female of 55 years age), the increase in lifetime incidence of cancer over baseline cancer risk was extremely small at 0.52% for 4DCT and 0.19% for scintigraphy. Given both studies cause negligible increases in lifetime risk of cancer, clinicians should not allow concern for radiation-induced cancer influence decisions regarding workup in older patients.

Dr Jenny Hoang, MBBS FRANZCR

Associate Professor of Radiology and Radiation Oncology at Duke University Medical Center in Durham NC, United States of America. Dr Hoang has particular expertise in the area of parathyroid imaging and has produced several published and in press papers related to this topic. Connect with her via twitter @JennyKHoang.

Comments

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